Scatterometry is currently used in the semiconductor industry to measure thickness and optical properties of thin films as well as the critical dimension (CD) and profile shape of periodic structures on a semiconductor wafer. In principle, scatterometry has clear advantages over the current imaging technology of overlay metrology. Scatterometry may be capable of measuring device-size structures that cannot be resolved by imaging. Scatterometry may be also thought to be more robust to process variations and asymmetry in the profile of the measured structure.
Methods for measuring profile asymmetry include critical dimension scanning electron microscopy (CD-SEM) and scatterometry. The CD-SEM approach may be very slow and expensive. The current implementation of scatterometry CD metrology, which may be also suitable for monitoring properties of the profile (including profile asymmetry), relies on detailed modeling and may be therefore also rather slow. In addition, it may be very difficult to accurately model complicated profiles, such as two gratings (one on top of the other) separated by a layered possibly non-flat film.